The invention relates generally to heat transfer and refrigeration control systems. More particularly, the invention relates to control devices particularly suited for detecting and controlling characteristics and mass flow of the working fluid in such systems.
The basic building blocks of all refrigeration and heat transfer systems are well known and include a compressor, a condenser, an expansion means and an evaporator, all of which are connected in a fluid circuit having a working fluid such as halogen containing refrigerants such as chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), and hydrofluorocarbons (HFCs), and so forth. In an automotive or truck air conditioning system, for example, the working fluid or refrigerant is typically in heat exchange with the vehicle compartment air by means of the evaporator. The liquid refrigerant turns to gas as it passes through the evaporator or endothermic heat exchanger thus absorbing heat from the ambient air. The gaseous refrigerant leaving the evaporator is drawn into the compressor through a suction line. The compressor increases the gas pressure and the gas then passes through the condenser or exothermic heat exchanger where it is cooled back to a liquid state but is still under high pressure. The pressurized liquid refrigerant is then passed through the expansion valve wherein the fluid pressure is adiabatically decreased prior to reentering the evaporator.
Over the years, many different types of control mechanisms and monitoring devices have been used to regulate the operation of heat transfer or refrigeration systems. One of the more important functions required of a heat transfer control system is to monitor and control the low pressure vapor quality in the suction line near the outlet of the evaporator or at the inlet to the compressor. This is important for many reasons, particularly to maximize cooling from the refrigerant flow, and to protect the compressor from receiving liquid refrigerant and/or a loss of lubricant.
A common technique in use today is to maintain a minimum superheat in the vaporous refrigerant exiting the evaporator. The superheat is usually maintained in the range of 5 to 10 degrees fahrenheit. In some systems the superheat is regulated by monitoring the evaporator inlet and outlet temperatures of the refrigerant and controlling the flow with the expansion valve so that the temperature difference is near a preset value or range. Other approaches include the use of pressure and temperature sensors on the outlet side of the evaporator to measure the actual saturation temperature and pressure characteristics of the refrigerant based on the thermodynamic properties of the refrigerant. Still another approach is the use of charged bulb sensors. From a heat transfer efficiency standpoint, it is desirable, of course, to maintain a low superheat which is difficult with the aforementioned sensors and controls.
Although these known approaches for regulating superheat can work, they tend to exhibit inaccurate control. One reason is that in the evaporator the liquid and gas phases are not in thermal equilibrium. The droplets of gas are boiling because heat is being transferred to the droplets from the gas phase. In order for this to take place, the gas must be hotter than the liquid, which makes conventional superheat measurement difficult.
Another significant problem with low superheat control systems is that mass flow of the refrigerant through the evaporator can change over a large range without an appreciable change in the conventionally measured superheat. Thus, superheat is a poor control mechanism for regulating the quality of the refrigerant at the outlet of the evaporator.
In addition to evaporator outlet quality, being able to determine the pressure conditions on the high pressure side of the expansion valve is also useful in maintaining efficient operation of the heat transfer system. In the past, pressure detection on the system high pressure side between the compressor and the expansion valve has been accomplished by such means as mechanical or electromechanical pressure transducers, pressure responsive valves or simpler temperature sensors, the latter being used to approximate pressure based on the ambient temperature of the refrigerant. The mechanically responsive pressure sensors and valves tend to exhibit slow response times to working fluid conditions, low reliability and limited control capabilities. More recently, the ready availability of electronic controllers such as microprocessors and other digital/analog controllers has provided the opportunity to electronically control and monitor the operation of the heat transfer system. This has an important benefit of being able to greatly reduce the cost, size and weight of the control system, as well as improving the reliability and flexibility of the control functions.
Although the use of electronic controllers is well known, a suitable electronic pressure sensor has not yet been realized that is low cost but reliable and simple to incorporate into both new refrigeration systems as well as for retrofitting or upgrading older systems. Past efforts, for example, have attempted to use self-heated thermistors to boil the refrigerant and thus determine the pressure based on the boiling point. This approach is inherently flawed, however, because a thermistor senses its own temperature, not the temperature of the refrigerant. By allowing the thermistor to self-heat by forcing constant current therethrough, the temperature measurement becomes inaccurate and unreliable. The refrigerant boiling point technique is further flawed by the fact that the controller can only be operated on the assumption that the thermistor is actually sensing the saturation temperature (i.e. the boiling point). The thermistor cannot detect the boiling event per se.
Accordingly, the need exists for an economical, reliable and accurate apparatus and method for detecting characteristics of a working fluid in a heat transfer system, particularly as those characteristics relate to controlling the quality of the refrigerant between the outlet of the evaporator and the compressor inlet, as well as determining and controlling pressure in the high pressure regions of the fluid circuit.